Jim Gaffigan was born in Elgin, Illinois, and raised in Chesterton, Indiana. He attended La Lumiere School in La Porte, Indiana. He is the youngest of six children and often jokes about growing up in a large family. He attended Purdue University for one year, where he was a member of the Phi Gamma Delta Fraternity. He graduated from Georgetown University's McDonough School of Business in 1988. After graduating, Gaffigan moved to NYC to work in advertising, taking improv classes at night. His comedy career began when a friend bet him to do stand-up.

He is married to actress Jeannie Noth, with whom he has two daughters, Marre and Katie Louise, and three sons, Jack, Michael, and Patrick. The family of seven live in a two-bedroom apartment in New York City. Gaffigan has stated on stage, and elsewhere, that he is Catholic.
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